1. Field of the Invention
The present invention relates to a device for generating wavelength dispersion and a device for compensating for wavelength dispersion accumulated in an optical fiber transmission line, and in particular, it relates to a device using a virtually-imaged phased array (VIPA) to generate wavelength dispersion.
2. Description of the Related Art
In a conventional fiber optic communication system for transmitting information using light, a transmitter sends a signal in the form of optical pulses to a receiver through an optical fiber. However, wavelength dispersion, which is also called “chromatic dispersion”, degrades the quality of the signals in the system.
More specifically, as a result of wavelength dispersion, the propagation speed of signal light in an optical fiber varies depending on the wavelength of the signal light. For example, if an optical pulse with a long wavelength (for example, optical pulse with a wavelength representing a red color) propagates faster than an optical pulse with a short wavelength (for example, optical pulse with a wavelength representing a blue color), the dispersion is called “normal dispersion”. Conversely, if an optical pulse with a short wavelength (for example blue pulse) propagates faster than an optical pulse with a long wavelength (for example, red pulse), the dispersion is called “abnormal dispersion”.
Therefore, if a signal optical pulse transmitted from a transmitter includes a red pulse and a blue pulse, the signal optical pulse is divided and separated into the red pulse and the blue pulse while they propagate through an optical fiber, and each of the respective optical pulses arrives at a different time at the receiver.
For another example of a signal optical pulse transmission, if a signal optical pulse with different wavelength components continuously ranging from a blue color to a red color is transmitted, each element propagates through an optical fiber at different speed. Therefore, the time width of the signal optical pulse is broadened in the optical fiber and distortion occurs. Since each pulse has elements in a limited wavelength range, such wavelength dispersion is very common in an fiber optic communication system.
Therefore, in particular, in a high-speed fiber optic communication system, in order to obtain high transmission capacity, such wavelength dispersion must be compensated for.
In order to compensate for such wavelength dispersion in an fiber optic communication system, a “reciprocal dispersion component” reversing the wavelength dispersion generated in an optical fiber to a signal optical pulse is needed.
Some conventional devices can be used for such a “reciprocal dispersion component”. For example, since a dispersion compensation fiber has a special sectional refractive index profile and applies a wavelength dispersion which is the reverse of wavelength dispersion generated in an ordinary transmission line fiber to a signal optical pulse, the fiber can be used as a “reciprocal dispersion component”. However, the production of a dispersion compensation fiber is expensive and a fairly long fiber is needed to sufficiently compensate for the wavelength dispersion generated in the transmission line fiber. For example, in order to completely compensate for wavelength dispersion generated in a 100 km transmission line fiber, a dispersion compensation fiber 20 km to 30 km long is needed. Therefore, optical loss increases and also dimensions become large, which are problems.
FIG. 1 shows a chirped fiber grating with a “reciprocal dispersion component” function to compensate for wavelength dispersion.
As shown in FIG. 1, light that suffers from wavelength dispersion when propagating through an optical fiber is supplied to the input port 48 of an optical circulator 50.
The optical circulator 50 supplies the light to the chirped fiber grating 52. The chirped fiber grating 52 returns the light to the circulator 50 so that each different wavelength component of the light can propagate over a different distance and wavelength dispersion can be compensated for.
For example, the chirped fiber grating 52 can be designed so that a long-wavelength component of light can be reflected over a long distance by the chirped fiber grating 52 and can propagate over a distance longer than a short-wavelength component of the light. Then, the circulator 50 supplies light reflected off the chirped fiber grating 52 to an output port 54. In this way, the chirped fiber grating 52 can apply reciprocal dispersion to a signal optical pulse.
However, the chirped fiber grating 52 has only too narrow a band to reflect a signal optical pulse. Therefore, a wavelength band sufficient to compensate for light containing many wavelengths such as wavelength division multiplexed light cannot be obtained. However, many chirped fiber gratings can be connected in a cascade for such a wavelength division multiplexed signal. In this case, the system costs dearly. The combination of a chirped fiber grating and a circulator shown in FIG. 1 is suitable for a one-channel fiber optic communication system.
FIG. 2 shows a conventional diffraction grating generating wavelength dispersion. As shown in FIG. 2, a diffraction grating 56 has a grated plane 58. A parallel ray 60 with different wavelengths is input to the grating plane 58. Then, the ray is reflected off each step of the grating plane 58, and the plurality of reflected rays interfere with one another. As a result, rays 62, 64 and 66 each with a different wavelength, are output from the diffraction grating 58 at different angles. In order to compensate for the wavelength dispersion of a parallel ray using a diffraction grating, the diffraction grating can be used in a spatial grating pair array, which is described in detail below.
FIG. 3A shows a spatial grating pair array used as a reciprocal dispersion component in order to compensate for wavelength dispersion. As shown in FIG. 3A, a parallel ray 67 is diffracted by the first diffraction grating 68 and is separated into a ray with a short wavelength 69 and a ray with a long wavelength 70. Then, these rays 69 and 70 are diffracted by the second diffraction grating 71 and propagate in the same direction. As shown in FIG. 3A, each wavelength component with a different wavelength propagates over a different distance, reciprocal dispersion is applied and wavelength dispersion is compensated for. Since a long wavelength like the ray 70 propagates over a distance longer than a short wavelength like the ray 69, the spatial grating pair array shown in FIG. 3A has abnormal dispersion.
FIG. 3B shows another spatial grating pair array used as a reciprocal dispersion component in order to compensate for chromatic dispersion. As shown in FIG. 3B, lenses 72 and 74 are inserted between the first and second diffraction gratings 68 and 71 so that the lenses 72 and 74 share one focal point. Since a long wavelength like the ray 70 propagates over a distance shorter than a short wavelength like the ray 69, the spatial grating pair array shown in FIG. 3B has normal dispersion.
The spatial grating pair arrays shown in FIGS. 3A and 3B are generally used as laser resonators to control dispersion. However, an actual spatial grating pair array cannot apply dispersion sufficient to compensate for a fairly large amount of chromatic dispersion generated in an fiber optic communication system. More specifically, angular dispersion generated by a diffraction grating is usually very small and is generally approximately 0.05 degree/nm. Therefore, in order to compensate for wavelength dispersion generated in an fiber optic communication system, the first and second diffraction gratings 68 and 71 must be located far apart. Therefore, such a spatial grating pair array is not practical.
Against the conventional device, Japanese Patent Application Nos. 10-534450 and 513133 propose a device comprising a virtually-imaged phased array (VIPA 1) shown in FIG. 4.
The VIPA 1 generates light that propagates from the VIPA 1. This device comprises a light returning device returning light to the VIPA 1 and generating multiple reflection in the VIPA 1.
The device can also comprise a VIPA receiving input light with a wavelength in a continuous wavelength range and generating a corresponding output light. This output light can be spatially distinguishable from output light with another wavelength in the continuous wavelength range (for example, each of them travels in a different direction). If this output light can be distinguishable by a traveling angle, it can be said that this device has angular dispersion.
Furthermore, such a device can also comprise a VIPA 1 and a light returning device 2. The VIPA 1 further comprises a transmission area and a transparent material. Light can be input/output to/from the VIPA 1 through the transmission area. The transparent material 3 has the first and second planes. The second plane has reflectivity and transmits part of the input light. The input light is received by the VIPA 1 through the transmission area, is reflected many times between the first and second planes of the transparent material. Then, a plurality of light rays are transmitted through the second plane. The plurality of transmitted light rays interfere with one another and output light 4 is generated. The input light has a wavelength in a continuous wavelength range, and the output light 4 can be distinguishable from another output light with another wavelength in the continuous wavelength range. The light returning device 2 can return the output light 4 to the second plane in the completely opposite direction, and the output light 4 is input to the VIPA 1 through the second plane. Then, the output light 4 is multiply reflected in the VIPA 1 and is output to an input route through the transmission areas of the VIPA 1.
Furthermore, such a device can also comprise a VIPA 1 generating a plurality of pieces of output light 4 with the same wavelength as that of input light but each with a different order of interference. This device also comprises the light returning device 2 returning some pieces of output light with one order of interference to the VIPA 1 and not returning the other pieces of output light. In this way, only light with one order of interference can be returned the VIPA 1.
Furthermore, such a device can also comprise a VIPA 1, a light returning device 2 and a lens 5. The VIPA 1 receives input light and generates corresponding output light that propagates from the VIPA 1. The light returning device 2 receives the output light 4 from the VIPA 1 and returns the output light to the VIPA 1. In the lens 5, (a) the light 4 output from the VIPA 1 transmits through lens 5, and travels to the light returning device 2 since they are collected at the light returning device 2 by the lens 5; then (b) the output light 4 travels from the light returning device 2 to the lens 5 and are returned from the light returning device 2 to the VIPA 1 since the output light is directed to the VIPA 1 by the lens 5; and further (c) the output light 4 traveling from the VIPA 1 to the lens 5 is positioned in such a way that the output light can travel in parallel with and in the opposite direction of the output light returned from the lens 5 to the VIPA 1 . Furthermore, output light traveling from the VIPA 1 to the lens 5 does not overlap output light returned from the lens 5 to the VIPA 1.
Furthermore, such a device can also comprise VIPAs 1, a mirror 6 and a lens 5. VIPA 1 receives input light and generates a corresponding output light that propagates from VIPA 1. The lens 5 collects output light rays 4 at the mirror 6, the mirror 6 reflects the output light rays and the reflected light rays are returned to the VIPA 1 by the lens 5. The mirror is shaped in such a way that the device can generate specific wavelength dispersion.
As described above, a VIPA has an angular dispersion function like a diffraction grating and can compensate for wavelength dispersion. In particular, the VIPA has large angular dispersion, and can easily provide a practical reciprocal dispersion component.
FIGS. 5 through 7 shows the characteristics of a device using a VIPA.
However, the transmittance characteristic by wavelength of a device using a VIPA to compensate for wavelength dispersion is not flat and is of round-top shape with a round peak around the center frequency of each transmission band, as shown in FIG. 5, which is another problem.
If there is a device with such a transmission characteristic in an optical transmission line, a signal optical pulse waveform transmitted from a transmitter is distorted and a signal cannot be accurately transmitted. In particular, if in a long-haul fiber optic communication system requiring the compensation of large wavelength dispersion, wavelength dispersion is compensated for by multi-connecting many devices using a VIPA, the undesirable transmission characteristics are overlapped and a signal optical pulse greatly degrades. In this case, it is preferable for a device using a VIPA to have a flat output light wavelength characteristic shown in FIG. 6.
Theoretically, such a non-flat transmission characteristic is generated in a device using a VIPA for the following reasons.
Firstly, the travel direction of output light from a VIPA varies depending on wavelength. In this case, light traveling in a direction in parallel with the axis of light input to the VIPA has the greatest strength and light traveling in a direction deviating from the axis of light input to the VIPA has less strength. This phenomenon can be explained as follows. Since light input to a VIPA usually consists of light rays converged by a lens, as shown in FIG. 4, the light contains a plurality of plane wave components each with a different travel direction. In this case, a plane wave component traveling in a direction in parallel with the axis of the input light has the greatest strength, and the greater the deviation from the axis of a plane wave component, the smaller is the strength of the plane wave component. However, a plurality of light rays multiply reflected in the VIPA interfere with one another and the wavelength of light, the strength of which is increased and output, varies for each plane wave component with a different travel direction. For this reason, it can be said that the strength of light output from a VIPA varies depending on wavelength.
This wavelength dependence of light output from a VIPA generates the wavelength dependence of transmittance in the transmission characteristic of a device using a VIPA. In other words, the transmission characteristic is not flat.
Secondly, since light with each wavelength output from a VIPA has a plurality of possible travel directions each with a different order of interference, the strength of the light is distributed among the plurality of pieces of light each with a different order of interference. In a device using a VIPA, light with an unnecessary order of interference must be discarded and light with a necessary order of interference must be extracted. Therefore, when light with an unnecessary order of interference to be discarded is output from the VIPA, there is optical loss corresponding to the unnecessary order of interference. However, whether light with each order of interference is output from the VIPA depends on the fact that the travel direction of the light with each order of interference is included in the travel directions of plane wave components of input light converged into the VIPA by a lens. Therefore, the number of times of the occurrence of light with an unnecessary order of interference varies depending on wavelength. The loss of light with a wavelength in which there is no light with an unnecessary order of interference is small, while the loss of light with a wavelength of light in which there are many unnecessary orders of interference is large. For this reason, the wavelength dependence of transmission wavelength characteristic is generated in a device using a VIPA. In other words, the transmission wavelength characteristic is not flat.
Thirdly, in order to compensate for wavelength dispersion by a device using a VIPA, an optical path length varying according to wavelength must be provided by returning light to a position varying according to wavelength when light output from the VIPA is reflected off a mirror and is returned to the VIPA. When light, the wavelength dispersion of which has been compensated for, is received and extracted as output light, the coupling efficiency of light to a fiber varies depending on wavelength due to the difference in a returning position in the VIPA depending on wavelength. Since usually an optical system is adjusted in such a way that light around the center wavelength of each transmission band can couple with a fiber most efficiently, the coupling efficiency of light with a wavelength away from the center wavelength is relatively low. For this reason, if wavelength dispersion is compensated for by a device using a VIPA, such a non-flat transmission characteristic can be obtained.
In Japanese Patent Application Nos. 10-534450 and 11-513133 described above, a detailed method for improving such a transmission characteristic and realizing such a preferable flat transmission characteristic in a device using a VIPA is not disclosed.
Using a conventional etalon filter, a characteristic the reverse of the wavelength characteristic of light output from a VIPA can be applied and the wavelength characteristic of the output light can be leveled to some degree. However, the transmission characteristic of a device using a VIPA sometimes has an asymmetrical shape with a peak wavelength as a center in which a wavelength width on a short wavelength side and a wavelength width on a long wavelength side are different in designing, as shown in FIG. 7, and it cannot be leveled in a strict sense. By multi-connecting etalon filters each with a different transmitted light cycle, an asymmetrical filter can be realized. However, the grater the number of filters, the greater the loss of transmitted light and the higher the cost of the system, which is not practical.